I have a few points of feedback on the current look and design of subitems. I am including a quick sketch below to illustrate which aspects I am highlighting and a proposed solution for each.

(1) Subitems Are Not Visually Connected to their Parent Items

Currently, subitems are floating in between parent items. If multiple subitems are expanded, this can make it difficult to see the visual hierarchy of the items/subitems even though they are indented slightly.

A simple line connecting them on the left side, may guide the eye more easily to each subitem. (See below)

(2) The Subitems Label Cannot Be Edited

The subitems label that currently sits above the subitems is somewhat distracting and can add to an already visually busy screen.

I would propose either removing it or allowing it to be customized so that it can meaningfully label whatever you place in the subitems. I have removed it in the example below for a cleaner look.

(3) Subitems are the Same Color as Parent Items

Again, because the subitems are the same color as the parent item, it can be difficult to distinguish the two types when looking at a board with multiple subitems in view

I would propose making the background color of each subitem a muted tone of the group color. (See below)